Previously, if the path.data setting listed multiple data paths, then a
shard would be “striped” across all paths by writing a whole file to each
path in turn (in accordance with the index.store.distributor setting). The
result was that files from a single segment in a shard could be spread across
multiple disks, and the failure of any one disk could corrupt multiple shards.

This striping is no longer supported. Instead, different shards may be
allocated to different paths, but all of the files in a single shard will be
written to the same path.

If striping is detected while starting Elasticsearch 2.0.0 or later, all of
the files belonging to the same shard will be migrated to the same path. If
there is not enough disk space to complete this migration, the upgrade will be
cancelled and can only be resumed once enough disk space is made available.